Reactor coolant pump is referred as “main pump”, which is one of the key equipment of a pressurized water reactor nuclear power plant, only the reliable operation of the main pump can ensure continuous and safe operation of nuclear power plants. Referring to FIG. 1, a shaft seal type main pump is provided with a water lubricated radial bearing 10 and a shaft seal assembly 11 on the pump body. The water lubricated radial bearing 10 mainly bears the radial force of the main pump; and the shaft seal assembly 11 ensures that the reactor coolant is prevented from leaking along the pump shaft 12 to the reactor plant under normal operating conditions and design basis accident conditions in a controllable leakage manner, and the shaft seal assembly 11 limits the amount of leakage to the limits specified in the accident safety analysis criteria at the SBO (Station Black Out) condition.
In order to ensure the cooling and lubrication of the water lubricated radial bearing 10 and the shaft seal assembly 11 and prevent the high temperature and high pressure coolant from being conducted to the shaft system above the impeller 13, so that the water lubricated radial bearing 10 and the shaft seal assembly 11 can be operated in an acceptable temperature range to ensure normal and stable operation of the main pump, it is necessary to provide a shaft seal water injection system on the pump body, and a redundancy design needs to be considered to improve its reliability. It is known that nuclear power plant shaft seal type main pump usually use a flange assembly without a heat exchanger provided therein. In order to ensure the normal operation of the main pump, the shaft seal water injection system is usually composed of two parts: upper filling water from the RCV system (Chemical and Volume Control System) and high temperature water from the main pump. The main pump is provided with an external heat exchanger which is supplied with equipment cooling water by the RRI system (Component Cooling Water System).
Referring to FIGS. 2 and 3, a related nuclear plant main pump shaft seal water injection system includes an jet pump 20, a hydrocyclone 21, a high pressure cooler 22 and a connection pipeline 23 provided outside the main pump, and two auxiliary pumps 24, 25 and an internal flow path provided inside the main pump. The inner and outer flow paths of the main pump are connected through a shaft seal water injection hole and a high temperature water drainage hole in the main flange 14. The working media of the jet pump 20 is a low temperature upper filling water from the RCV system and the ejection media is a high temperature reactor coolant at the back of the impeller 13 in the main pump, both of which are mixed in the jet pump 20, and then flows through the shaft seal water injection hole of the main flange 14 into the shaft seal chamber 15 after cooled by the high pressure cooler 22 and filtered by the hydrocyclone 21. One part of the injected water provides cooling and lubrication for the shaft seal assembly 11, and the other part of the injected water flows along the pump shaft 12 under the action of the auxiliary pump 24 to cool and lubricate the water lubricated radial bearing 10. The pressure is increased by the action of the other auxiliary pump 25 and then divided into two parts, one part of which enters the RCP system (Reactor Coolant system) and the other part enters the external circulation as the ejection media of the jet pump 20. The high pressure cooler 22 is provided with cooling water by the RRI system. The shaft seal water injection system works as follows: 1) the upper filling water of the RCV system is mixed with the high temperature reactor coolant as a shaft seal injection water normally; 2) when the injection water of the RCV system is lost and the main pump is running, a certain amount of water is circulated in the system under the action of the two auxiliary pumps 24, 25 to provide lubrication and cooling for the shaft seal assembly 11 and the water lubricated radial bearing 10; 3) when the main pump is out of service and the low temperature upper filling water of the RCV is lost, a natural circulation can be established under the action of the high pressure cooler 22 and a certain flow rate of the shaft seal injection water is continued; 4) when RRI equipment cooling water is lost and RCV low temperature upper filling water is normal, the temperature of the shaft seal injection water is within the allowable operating temperature range of the shaft seal assembly 11 and the water lubricated radial bearing 10.
However, the shaft seal water injection system shown in FIGS. 2 and 3 has at least the following drawbacks: first, the effective exhaust cannot be achieved and the normal operation of the system cannot be ensured; second, two auxiliary pumps 24, 25 are required in the main pump, which will increase the complexity of the internal structure of the main pump; third, the reactor coolant cannot be effectively isolated when the jet pump 20, the hydrocyclone 21 and the high pressure cooler 22 need to be serviced; fourth, the system status cannot be monitored because there is no corresponding monitoring instruments on the system loop; fifth, when the high pressure cooler 22 fails, the main pump must be stopped.
Referring to FIGS. 4 and 5, another related main pump shaft seal water injection system of a nuclear plant includes a jet pump 30, a high pressure cooler 31, a hydrocyclone 32, a number of valves, a connection pipeline 33, and a pressure sensor 34 provided outside the main pump, and an auxiliary pump 35 and an internal flow path provided inside the main pump. The inner and outer flow paths of the main pump are connected through a shaft seal water injection hole and a high temperature water drainage hole in the main flange 14a. The working media of the jet pump 30 is a low temperature upper filling water from the RCV system and the ejection media is a high temperature reactor coolant from the main pump housing 16, both of which are mixed in the jet pump 30 and then flows through the shaft seal water injection hole of the main flange 14a into the shaft seal chamber 15 after cooled by the high pressure cooler 31 and filtered by the hydrocyclone 32. One part of the injected water provides cooling and lubrication for the shaft seal assembly 11, and the other part of the injected water flows along the pump shaft 12 under the action of the auxiliary pump 35 to cool and lubricate the water lubricated radial bearing 10 and finally into the back of the main pump impeller 13 and confluences with the reactor coolant in the pump housing 16. The valve 36 on the main connection pipeline is in the normally open state, the valves 37 and 38 on the exhaust pipeline are in the normally closed state, and the valves 37 and 38 are used only for exhausting and are closed after completion of the exhaust. The shaft seal water injection system works as follows: 1) the valve 39 and the valve 40 are closed normally, and the low temperature water from the RCV system is injected directly into the shaft seal chamber 15 through the jet pump 30, the high pressure cooler 31 and the hydrocyclone 32 and acts as a main pump shaft seal injection water; the high pressure cooler 31 is in the stand-by state and the loss of the RRI apparatus cooling water does not affect the normal operation of the main pump; 2) when the low temperature injection water of the RCV system is lost and the main pump is running, the valve 40 is opened, the high temperature reactor coolant from the main pump housing 16 is cooled by the high pressure cooler 31 and flows through the hydrocyclone 32 into the shaft seal chamber 15, so as to provides lubrication and cooling for the shaft seal assembly 11 and the water lubricated radial bearing 10; when the main pump is shut down, a natural circulation can be established under the action of the high pressure cooler 31 to continue supplying the shaft seal injection water for the main pump; 3) when the main pump is out of service and the valve 39 is opened, the low temperature water from the RCV system and the high temperature reactor coolant from the pump housing 16 are mixed in the jet pump 30 and then passed through the high pressure cooler 31 and the hydrocyclone 32 into the shaft seal chamber 15; when the main pump is shut down, a natural circulation can be established under the action of the high pressure cooler 31 to continue supplying the shaft seal injection water for the main pump.
However, the shaft seal water injection system as shown in FIGS. 4 and 5 at least has the following disadvantages: first, the control of the seal water injection system in running is complex, valves 39, 40 need to be operated usually; second, when the main pump is out of service, if the upper filling water of RCV is lost at the same time, a natural circulation cannot be established, which may lead to failure of providing shaft seal injection water; third, when the high pressure cooler 31 and the jet pump 30 is out of service, the main pump must be stopped; fourth, the valve 36 is located on the connection pipeline 33 before the shaft seal water injection hole of the main flange 14a, if the valve 36 malfunctions, normal operation of the shaft seal water injection system will be affected.
In view of the foregoing, what is needed, therefore, is to provide a main pump shaft seal water injection system of nuclear power plant which can solve the above problems.